The invention relates to a respiration-controlled cardiac pacemaker.
A cardiac pacemaker is known from published European Patent Specification No. 89 014, the basic stimulating pulse rate of which is controlled as a function of respiration, in order to be able to adapt the stimulation rate more appropriately to the physiological needs of the patient. A rheography pulse generator of the pacemaker generates, at a constant frequency, successive constant current pulses which flow between two electrodes implanted at a distance from each other in the ribcage of a patient. The voltage between the two electrodes is proportional to the ribcage impedance and varies as a function of respiration as the ribcage expands. A respiration detector, responding to the voltage amplitude of the rheography pulses, provides, for instance by time-related integration of the pulse-voltage amplitudes, a respiration signal which corresponds to the voltage fluctuation and hence the rate of respiration. Control means control the basic stimulation rate of the cardiac pacemaker as a function of the respiration signal in accordance with a predetermined respiration signal--basic stimulation rate characteristic, particularly in that the basic stimulation rate is raised with increasing respiration rate.
In the case of the known cardiac pacemaker, the frequency of the rheography pulses is considerably greater than the basic stimulation rate. This can lead to stimulation disturbances, because the cardiac pacemaker is, in certain circumstances, inhibited by its own rheography pulses. Furthermore, ECG measurements are made more difficult because the rheography pulses appear in the ECG picture and make it difficult to monitor the cardiac pacemaker function on the patient. Furthermore, under conditions of hyperventilation which are linked with a sudden increase in respiration rate, the increase in basic stimulation rate which is undesirable in such a case cannot be prevented.
The object of the invention is to improve a respiration-controlled cardiac pacemaker of the previously described type so that disturbances of the ECG picture caused by rheography pulses are kept within negligible limits.